Color-stable superabsorbent polymer composition

ABSTRACT

A color-stable superabsorbent polymer having long-term color stability, and methods of manufacturing the polymer, are disclosed. The superabsorbent polymer is prepared using a sulfinic acid derivative, like 2-hydroxy-2-sulfinatoacetic acid, a salt thereof, or a mixture thereof, as the reducing agent in a polymerization initiator system for the preparation of a superabsorbent polymer from monomers. The resulting superabsorbent polymer resists color degradation during periods of extended storage, even at an elevated temperature and humidity.

CROSS-REFERENCE TO RELATED APPLICATION

This is the U.S. national phase application of International ApplicationNo. PCT/EP2004/002874, filed Mar. 19, 2004, which claims the benefit ofU.S. provisional patent application Ser. No. 60/457,841, filed Mar. 26,2003.

FIELD OF THE INVENTION

The present invention relates to superabsorbent polymers (SAPS) havinglong-term color stability, and to methods of preparing the color-stableSAPs. More particularly, the present invention relates to methods ofpreparing a color-stable SAP using a monomer mixture containing apolymerization initiator comprising a sulfinic acid derivative, such as2-hydroxy-2-sulfinatoacetic acid, a salt thereof, or a mixture thereof,and, optionally, subjecting the resulting SAP hydrogel to a low dose ofUV radiation. The color-stable SAP can be incorporated into articles,such as bandages, diapers, sanitary napkins, and other absorbentproducts, wherein the SAP retains a clean, white color during extendedstorage periods, even under high temperature and humidity conditions.

BACKGROUND OF THE INVENTION

Water-absorbing resins are widely used in sanitary and hygienic goods,wiping cloths, water-retaining agents, dehydrating agents, sludgecoagulants, disposable towels and bath mats, disposable door mats,thickening agents, disposable litter mats for pets,condensation-preventing agents, and release control agents for variouschemicals. Water-absorbing resins are available in a variety of chemicalforms, including substituted and unsubstituted natural and syntheticpolymers, such as hydrolysis products of starch acrylonitrile graftpolymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonatedpolystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols,polyethylene oxides, polyvinylpyrrolidones, and polyacrylonitriles.

Such water-absorbing resins are termed “superabsorbent polymers,” orSAPs, and typically are lightly crosslinked hydrophilic polymers. SAPsare discussed generally in U.S. Pat. Nos. 5,669,894 and 5,559,335, eachincorporated herein by reference. SAPs can differ in their chemicalidentity, but all SAPs are capable of absorbing and retaining amounts ofaqueous fluids equivalent to many times their own weight, even undermoderate pressure. For example, SAPs can absorb one hundred times theirown weight, or more, of distilled water. The ability to absorb aqueousfluids under a confining pressure is an important requirement for an SAPused in a hygienic article, such as a diaper.

As used herein, the term “SAP particles” refers to superabsorbentpolymer particles in the dry state, more specifically, particlescontaining from no water up to about 10%, by weight, water. The terms“SAP gel,” “SAP hydrogel,” or “hydrogel” refer to a superabsorbentpolymer containing at least about 10%, by weight, water, and typically,particles that have absorbed at least their weight in water, and moretypically several times their weight in water.

SAPs have a tendency to degrade in color after long periods of storage.The tendency of an SAP to undergo a color transition from a clean,crisp, white color to a honey brown color accelerates as storage time,temperature, and humidity increase. In temperate climates, such as theUnited States and Europe, the rate at which an SAP undergoes colordegradation is sufficiently slow such that the SAP, or articlecontaining the SAP, typically is consumed before a color change isobservable to the naked eye.

However, in tropical and subtropical climates, such as in South Americaand Southeast Asia, SAP color degradation is sufficiently rapid suchthat a color change often occurs before the SAP, or article containingthe SAP, is consumed. In areas like Southeast Asia, an SAP can changecolor from white to honey brown in about 4 to 6 weeks. This problem isexacerbated because the SAPs may be produced far from the tropicalclimate, thereby increasing the time span from SAP production to use.Furthermore, consumption of articles containing an SAP in such climatesis relatively low, therefore further increasing the time period betweenSAP production and use.

The change in color of the SAP does not affect SAP performance, butadversely affects consumer acceptance of articles containing thecolor-degraded SAPs. In particular, consumers observing a color-degradedSAP in a diaper form an opinion that the diaper contains a contaminant,is somehow soiled or faulty, or is of low quality. The diaper typicallyis returned for a refund, and the consumer is less likely to repurchasethat brand of diaper.

Problems also arise at the manufacturing level because manufacturers ofdiapers and other articles containing an SAP refuse to incorporate adiscolored SAP into their products, and return the discolored SAP to theSAP manufacturer. A color-degraded SAP, therefore, ultimately adverselyaffects the manufacturer of articles and the manufacturer of the SAP,who must absorb the cost of the returned goods.

It would be desirable to provide an SAP that exhibits exceptional colorstability properties, such that the SAP retains its crisp, white colorthroughout the useful life of the SAP, or an article containing the SAP,even when stored under high temperature and humidity conditions.Furthermore, it would be desirable to provide an SAP having a long-termcolor stability and low residual monomer content, without adverselyaffecting the absorbent properties of the SAP, such as absorbing a largeamount of liquids quickly, having a good fluid permeability into andthrough the SAP, and having a high gel strength, such that an SAPhydrogel formed from the SAP does not deform or flow under an appliedstress or pressure.

Currently, SAPs, like partially neutralized, lightly crosslinked,polyacrylic acid, are manufactured using a persulfate as a component ofthe polymerization initiator system. A persulfate is included in theinitiator system as the oxidizing agent of a redox initiator pair and toreduce the amount of residual acrylic acid monomer in the SAP toacceptable levels. A persulfate also can act as a thermal initiator.However, the persulfate further interacts with the MEHQ inhibitorpresent in acrylic acid monomer and imparts a low initial color to theSAP. This low initial SAP color progresses to a severe SAP discolorationover time, and especially under high temperature and humidityconditions.

The present invention is directed to overcoming the problem of SAPdiscoloration attributed to the presence of a color-producing oxidizingagent, like a persulfate, in the preparation of an SAP. As discussed indetail hereafter, the present invention overcomes the SAP discolorationproblem (a) by utilizing a polymerization initiator system comprising asulfinic acid derivative, such as 2-hydroxy-2-sulfinatoacetic acid,optionally 2-hydroxy-2-sulfonatoacetic acid, salts thereof, or a mixturethereof, (b) by essentially omitting a color-producing oxidizing agentfrom the monomer mixture, and, (c) optionally, by subjecting the SAPhydrogel resulting from the polymerization to a low dose of ultraviolet(UV) radiation.

2-Hydroxy-2-sulfinatoacetic acid disodium salt and2-hydroxy-2-sulfonatoacetic acid disodium salt have been used as areducing agent in a redox initiator in emulsion polymerizations. U.S.Pat. No. 5,408,019 discloses using formamidine sulfonic acid as thereducing agent in a redox initiator system.

Ultraviolet radiation previously has been used in the preparation ofSAPs. For example, UV radiation has been used in conjunction with aphotoinitiator to initiate polymerization of monomers and provide an SAPhydrogel, as disclosed in EP 0 290 814 B1. DE 41 23 889 A1 discloses UVirradiation of a water-absorbing resin prepared from a water-solublepolymer and a polysaccharide and/or crosslinking agent, in the presenceof a radical scavenger, to provide a water-absorbing resin having a lowαmount of water-soluble components (≦7 wt %) and a low amount ofresidual monomer (≦500 ppm). The UV radiation is applied during dryingor crushing of the water-absorbing resin.

PCT publication WO 01/55228 discloses subjecting a water-soluble orwater-swellable polymer to UV radiation to reduce residual monomercontent. An ultraviolet initiator is used in an amount of up to 10,000ppm, by weight of monomers, preferably up to 5000 ppm, more preferably50 to 3,000 ppm, and still more preferably 500 to 2,000 ppm. UVradiation typically is conducted for about 20 minutes.

PCT publication WO 01/25289 discloses subjecting an acrylic polymer toUV radiation after, or simultaneously with, comminuting a gelled polymerto gelled polymer particles.

In particular, the comminuted gel particles can be irradiated during adrying step in a fluid bed dryer.

SUMMARY OF THE INVENTION

The present invention is directed to a superabsorbent polymer (SAP)having long-term color stability, and to methods of manufacturing acolor-stable SAP composition. More particularly, the present inventionis directed to a method of preparing a color-stable SAP, withoutadversely affecting the fluid absorption and retention properties of theSAP particles, (a) by utilizing a polymerization initiator systemcomprising a sulfinic acid derivative, such as2-hydroxy-2-sulfinatoacetic acid, optionally 2-hydroxy-2-sulfonatoaceticacid, salts thereof, or mixtures thereof, (b) by essentially omitting acoloring-forming oxidizing agent, like a persulfate, from the monomermixture, and, optionally, (c) by subjecting the SAP hydrogel resultingfrom the polymerization to a low dose of UV radiation. A color-stableSAP prepared by the present method retains a crisp, clean white colorover an extended storage period at a high temperature and humidity,i.e., at least 30 days when stored at 60° C. and 90% relative humidity.

One aspect of the present invention, therefore, is to provide a methodof manufacturing a color-stable SAP, including the steps of (a)polymerizing a monomer mixture comprising (i) a monomer that provides anSAP, like an α,β-unsaturated carboxylic acid, such as acrylic acid,either neutralized, unneutralized, or partially neutralized, (ii) acrosslinking agent, (iii) an initiator system comprising a sulfinic acidderivative, such as 2-hydroxy-2-sulfinatoacetic acid, a salt thereof, ora mixture thereof, and that is essentially free of color-producingoxidizing agent, e.g., a persulfate, and (ivy an optionalphotoinitiator, to form an SAP hydrogel, (b) optionally subjecting theSAP hydrogel to a low dose of UV radiation, and (c) comminuting anddrying the SAP hydrogel to provide a color-stable SAP. The resultingcolor-stable SAP has a low residual monomer content and maintains acrisp white color over an extended time, even under high temperature andhumidity storage conditions.

Another aspect of the present invention is to provide a method ofmanufacturing a color-stable SAP including the steps of polymerizing amonomer mixture comprising a sulfinic acid derivative, such as2-hydroxy-2-sulfinatoacetic acid, a salt thereof, or a mixture thereof,that provides an SAP, for example, a polymerized α,β-unsaturatedcarboxylic acid, to form an SAP hydrogel, optionally subjecting the SAPhydrogel to UV radiation for about 3 to about 15 minutes from a distanceof about 2 to about 30 centimeters, or an equivalent UV radiation dose,comminuting the SAP hydrogel to form SAP hydrogel particles, then dryingthe SAP hydrogel particles to provide color-stable SAP particles. TheSAP hydrogel optionally is subjected to a low dose of UV radiation,i.e., 0 to about 2000 milliwatts (mW) of UV radiation per squarecentimeter (cm²) of SAP hydrogel. In another embodiment, the SAPhydrogel is comminuted to form SAP hydrogel particles before beingsubjected to UV radiation.

A sulfinic acid derivative useful in the present invention has a generalstructural formula:

wherein M is hydrogen, an ammonium ion, or a monovalent or a divalentmetal ion of groups Ia, IIa, IIb, IVa, and VIIIb of the Periodic Tableof the Elements;

-   R¹ is OH or NR⁴R⁵, wherein R⁴ and R⁵, independently, are H or    C₁-C₆alkyl;-   R² is H or an alkyl, alkenyl, cycloalkyl, or aryl group, optionally    having 1-3 substituents independently selected from the group    consisting of C₁-C₆alkyl, OH, O—C₁-C₆alkyl, halogen, and CF₃; and-   R³ is COOM, SO₃M, COR⁴, CONR⁴R⁵, or COOR⁴, wherein M, R⁴, and R⁵ are    as defined above, or, if R² is unsubstituted or substituted aryl, R³    is H;    and the salts thereof.

In preferred embodiments, the monomer mixture comprises (a) anα,β-unsaturated carboxylic acid, (b) a crosslinking agent, (b) apolymerization initiator system comprising 2-hydroxy-2-sulfinatoaceticacid, a salt thereof, or a mixture thereof, that is essentially free ofa persulfate and other color-producing oxidizing agents, (d) an optionalphotoinitiator in an amount of 0 to about 1000 ppm by weight ofα,β-unsaturated carboxylic acid and crosslinking agent, and (e) water.After drying, the color-stable SAP particles optionally are surfacetreated to provide surface crosslinks on the color-stable SAP particles.

Yet another aspect of the present invention is to incorporate thecolor-stable SAP particles into articles used to absorb liquids, forexample, a diaper, a catamenial device, a feminine hygiene product, anadult incontinence product, general purpose wipes and cloths, andsimilar absorbent products. The articles resist color degradation overthe expected life of the article, even in high temperature and humidityclimates.

The above and other aspects and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments of the invention, taken in conjunction with the examples andthe claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to SAPs having a long-term colorstability, and to methods of preparing the color-stable SAPs. Thecolor-stable SAPs show a minor color change, to the naked eye, afterstorage for 30 days at 60° C. and 90% relative humidity. The presentSAPs are prepared from a monomer mixture containing a polymerizationinitiator comprising a sulfinic acid derivative, such as2-hydroxy-2-sulfinatoacetic acid, a salt thereof, or a mixture thereof,that is essentially free of a color-forming oxidizing agent, like apersulfate. The SAP hydrogel resulting from polymerizing the monomermixture optionally is subjected to a low dose of UV radiation. Thepresent polymerization initiator, and optional UV radiation, impartcolor stability to the SAP and reduce the amount of residual monomer inthe SAP.

Suitable processes for preparing the present color-stable SAP particlesinclude solution polymerization, also termed gel polymerization, forexample, as disclosed in U.S. Pat. Nos. 4,076,663; 4,286,082; 4,654,039;and 5,145,906, each incorporated herein by reference. Another process isinverse suspension polymerization disclosed, for example, in U.S. Pat.Nos. 4,340,706; 4,497,930; 4,666,975; 4,507,438; and 4,683,274, eachincorporated herein by reference.

The present disclosure is directed primarily to gel polymerization forillustrative purposes. However, the invention can be practiced using allSAP manufacturing processes, including modified bulk polymerization andinverse suspension polymerization.

In gel polymerization, SAPs are prepared from an aqueous mixture ofmonomers and one or more crosslinking agents to provide awater-absorbent, but water-insoluble, polymer. The aqueous monomermixture also contains polymerization initiators, typically including apersulfate, like sodium persulfate. A persulfate has been considered animportant or essential polymerization initiator ingredient in order toreduce the residual acrylic acid monomer content in SAP particles toacceptable levels.

In the typical manufacture of an SAP, the SAP is neutralized at leastabout 25 mole percent, more preferably at least about 50 mole percent,and usually about 70 to about 80 mole percent, to achieve optimumabsorbency. Neutralization can be achieved by neutralizing the monomersbefore polymerization, or the polymer can be neutralized after thepolymerization reaction is substantially complete. After polymerizationand internal crosslinking of the monomers, followed by partialneutralization, e.g., about 50 to about 100 mole percent neutralization,preferably about 70 to about 80 mole percent neutralization, the polymeris comminuted, e.g., shredded or chopped, for more efficient drying,then dried and milled to a desired particle size. The polymer preferablythen is surface treated. In embodiments wherein surface treatment isemployed, a surface crosslinking agent typically is applied to thedried. SAP particles. Generally, after application of the surfacecrosslinking agent, the SAP particles then are subjected to conditionswherein the surface crosslinking agent reacts with a portion of the SAPto crosslink the surfaces of the SAP particles.

In one embodiment of the present invention, a color-stable SAP isprepared by a method comprising the steps of (a) solution polymerizing amonomer mixture comprising (i) a monomer capable of providing an SAPpolymer, like an α,β-unsaturated carboxylic acid, such as acrylic acid,either neutralized, unneutralized, or partially neutralized, (ii) acrosslinking agent, (iii) a polymerization initiator comprising asulfinic acid derivative, such as 2-hydroxy-2-sulfinatoacetic acid, asalt thereof, or a mixture thereof, that is essentially free of acolor-forming oxidizing agent, like a persulfate, (iv) an optionalphotoinitiator, and (v) water, to form an SAP hydrogel, (b) optionallysubjecting the SAP hydrogel to a low dose of UV radiation, (c)comminuting the SAP hydrogel to form SAP hydrogel particles, (d) thendrying the resulting SAP hydrogel particles, and (e) optionally surfacetreating the color-stable SAP particles.

The present color-stable SAPs are based on polymerized vinyl monomers,particularly α,β-unsaturated carboxylic acids, that, afterpolymerization, have the ability to absorb several times their weight ofa liquid when crosslinked. The remainder of the specification isdirected to a color-stable SAP based on acrylic acid, however, othervinyl monomers, like (meth)acrylonitrile or a (meth)acrylamide, or anethylenic monomer having an amine substituent or a precursor to an aminesubstituent, e.g., N-vinyl acetamide, and other α,β-unsaturatedcarboxylic acids and anhydrides, also can be used in the manufacture ofcolor-stable SAPs of the present invention. The color-stable SAPsprepared by the present methods exhibit improved color stabilityregardless of the identity of the monomers used to prepare the SAP, andparticularly SAPs based on an α,β-unsaturated carboxylic acid oranhydride.

Accordingly, neither the chemical makeup of the color-stable SAP, norits method of manufacture, is limited. The color-stable SAPs, therefore,can be prepared by any SAP polymerization process known in the art andcan comprise an acidic water-absorbing resin (i.e., an anionic SAP), abasic water-absorbing resin (i.e., a cationic SAP), or a multicomponentSAP particle as disclosed in U.S. Pat. Nos. 6,072,101; 6,159,591;6,222,091; and 6,329,062, each incorporated herein by reference. Anextensive list of suitable SAP-forming monomers can be found in U.S.Pat. Nos. 4,076,663 and 5,149,750, each incorporated herein byreference.

Generally, acidic SAPs have carboxylate, sulfonate, sulfate, and/orphosphate groups incorporated along the polymer chain. Polymerscontaining these acid moieties are synthesized either from monomerspreviously substituted with one or more of these acidic functionalgroups or by incorporating the acidic functional group into the polymerafter synthesis. To incorporate carboxyl groups into a polymer, any of anumber of ethylenically unsaturated carboxylic acids can behomopolymerized or copolymerized. Carboxyl groups also can beincorporated into the polymer chain indirectly by hydrolyzing ahomopolymer or copolymer of monomers such as acrylamide, acrylonitrile,methacrylamide, and alkyl acrylates or methacrylates. An acidic SAP canbe either a strong or a weak acidic water-absorbing resin, and can be ahomopolymer or a copolymer.

The acidic SAP typically is a neutralized, lightly crosslinkedacrylic-type resin, such as neutralized, lightly crosslinked polyacrylicacid. The lightly crosslinked acidic SAP typically is prepared bypolymerizing an acidic monomer containing an acyl moiety, e.g., acrylicacid, or a moiety capable of providing an acid group, i.e.,acrylonitrile, in the presence of a crosslinking agent, i.e., apolyfunctional organic compound. The acidic resin can contain othercopolymerizable units, i.e., other monoethylenically unsaturatedcomonomers, well known in the art, as long as the polymer issubstantially, i.e., at least 10%, and preferably at least 25%, acidicmonomer units. To achieve the full advantage of the present invention,the acidic SAP contains at least 50%, and more preferably, at least 75%,and up to 100%, acidic monomer units. The acidic resin can beunneutralized or neutralized, preferably neutralized at least 50 mole %,and most preferably at least 70 mole %, with a base prior to drying.

Ethylenically unsaturated carboxylic acid monomers, and anhydrides,amides, esters, and salts thereof, useful in the acidic SAP include, butare not limited to, acrylic acid, methacrylic acid, ethacrylic acid,α-chloroacrylic acid, α-cyanoacrylic acid, β-methylacrylic acid(crotonic acid), α-phenylacrylic acid, β-acryloxypropionic acid, sorbicacid, α-chlorosorbic acid, angelic acid, cinnamic acid, p-chlorocinnamicacid, β-stearylacrylic acid, itaconic acid, citraconic acid, mesaconicacid, glutaconic acid, aconitic acid, maleic acid, fumaric acid,tricarboxyethylene, 2-methyl-2-butene dicarboxylic acid, maleamic acid,N-phenyl maleamide, maleamide, maleic anhydride, fumaric anhydride,itaconic anhydride, citraconic anhydride, mesaconic anhydride, methylitaconic anhydride, ethyl maleic anhydride, diethyl maleate, methylmaleate, and maleic anhydride.

Sulfonate-containing acidic SAPs can be prepared from monomerscontaining functional groups hydrolyzable to the sulfonic acid form, forexample, alkenyl sulfonic acid compounds and sulfoalkyl acrylatecompounds. Ethylenically unsaturated sulfonic acid monomers include, butare not limited to, aliphatic or aromatic vinyl sulfonic acids, such asvinylsulfonic acid, allylsulfonic acid, vinyltoluene sulfonic acid,styrenesulfonic acid, acrylic and methacrylic sulfonic acids, such assulfoethyl acrylate, sulfoethyl methacrylate, sulfopropyl acrylate,2-vinyl-4-ethylbenzenesulfonic acid, 2-allylbenzenesulfonic acid,1-phenylethylene sulfonic acid, sulfopropyl methacrylate,2-hydroxy-3-methacryloxypropyl sulfonic acid, and2-acrylamide-2-methylpropane sulfonic acid.

Sulfate-containing acidic SAPs are prepared by reacting homopolymers orcopolymers containing hydroxyl groups or residual ethylenic unsaturationwith sulfuric acid or sulfur trioxide. Examples of such sulfatedpolymers include sulfated polyvinyl alcohol, sulfated hydroxyethylacrylate, and sulfated hydroxypropyl methacrylate. Phosphate-containingacidic SAPs are prepared by homopolymerizing or copolymerizingethylenically unsaturated monomers containing a phosphoric acid moiety,such as methacryloxy ethyl phosphate.

The acidic SAP, either strongly or weakly acidic, can be any resin thatacts as an SAP in its neutralized form. Examples of acidic resinsinclude, but are not limited to, polyacrylic acid, hydrolyzedstarch-acrylonitrile graft copolymers, starch-acrylic acid graftcopolymers, saponified vinyl acetate-acrylic ester copolymers,hydrolyzed acrylonitrile copolymers, hydrolyzed acrylamide copolymers,ethylene-maleic anhydride copolymers, isobutylene-maleic anhydridecopolymers, poly(vinylsulfonic acid), poly(vinylphosphonic acid),poly(vinylphosphoric acid), poly(vinylsulfuric acid), sulfonatedpolystyrene, poly(aspartic acid), poly(lactic acid), and mixturesthereof. The preferred acidic resins are the polyacrylic acids.

The acidic SAP contains 0 to 100 percent neutralized pendant carboxylategroups (i.e., DN=0 to DN=100). Neutralization of carboxylic acid groupsis accomplished using a strong organic or inorganic base, such as sodiumhydroxide, potassium hydroxide, ammonia, ammonium hydroxide, or anorganic amine.

Analogous to the acidic SAP, a color-stable basic SAP can bemanufactured by the present method. The basic SAP can be a strong orweak basic water-absorbing resin. The strong basic resins typically arepresent in the hydroxide (OH) or bicarbonate (HCO₃) form. The basic SAPcan be a single resin or a mixture of resins. The basic SAP can be ahomopolymer or a copolymer.

The basic SAP, either strongly or weakly basic, therefore, can be anyresin that acts as an SAP in its charged form. The basic SAP typicallyis a lightly crosslinked resin, such as a lightly crosslinkedpolyethylenimine, a poly(allylamine), a poly(allylguanidine), apoly(dimethyldiallylammonium hydroxide), a quaternized polystyrenederivative, a guanidine-modified polystyrene, a poly(vinylguanidine), ora poly(dialkylaminoalkyl (meth)acrylamide) prepared by polymerizing andlightly crosslinking a monomer having the structure

or its ester analog

wherein R₇ and R₈, independently, are selected from the group consistingof hydrogen and methyl, Y is a divalent straight chain or branchedorganic radical having 1 to 8 carbon atoms, R₉ is hydrogen, and R₁₀ ishydrogen or an alkyl radical having 1 to 4 carbon atoms.

Preferred basic SAPs include a poly(vinylamine), polyethylenimine,poly(vinylguanadine), poly(methylaminoethyl acrylamide),poly(methylaminopropyl methacrylamide), or mixtures thereof. Basic SAPsare disclosed in U.S. Pat. No. 6,159,591, incorporated herein byreference. The lightly crosslinked basic SAP can contain othercopolymerizable units and is crosslinked using a polyfunctional organiccompound, as set forth above with respect to the acidic SAP.

Copolymerizable monomers for introduction into an acidic SAP or a basicSAP include, but are not limited to, ethylene, propylene, isobutylene,C₁₋₄ alkyl acrylates and methacrylates, vinyl acetate, methyl vinylether, and styrenic compounds having the formula:

wherein R represents hydrogen or a C₁₋₆ alkyl group, and wherein thephenyl ring optionally is substituted with one to four C₁₋₄ alkyl orhydroxy groups.

Suitable C₁₋₄ alkyl acrylates include, but are not limited to, methylacrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butylacrylate, and the like, and mixtures thereof. Suitable C₁₋₄ alkylmethacrylates include, but are not limited to, methyl methacrylate,ethyl methacrylate, isopropyl methacrylate, n-propylmethylmethacrylate,n-butyl methacrylate, and the like, and mixtures thereof or with C₁₋₄alkyl acrylates. Suitable styrenic compounds include, but are notlimited to, styrene, α-methylstyrene, p-methylstyrene, t-butyl styrene,and the like, and mixtures thereof or with C₁₋₄ alkyl acrylates and/ormethacrylates.

As previously stated, the present invention is not limited to SAPs basedon acrylic acid, but preferably extends to SAPs prepared forα,β-unsaturated carboxylic acids including, but not limited to,methacrylic acid, ethacrylic acid, α-chloroacrylic acid, α-cyanoacrylicacid, β-methylacrylic acid (crotonic acid), α-phenylacrylic acid,β-acryloxypropionic acid, sorbic acid, α-chlorosorbic acid, angelicacid, cinnamic acid, p-chlorocinnamic acid, β-stearylacrylic acid,itaconic acid, citraconic acid, mesaconic acid, glutaconic acid,aconitic acid, maleic acid, fumaric acid, tricarboxyethylene, and maleicanhydride. Acrylic acid, i.e., CH₂═CHCO₂H, is the most preferredα,β-unsaturated carboxylic acid.

Especially preferred SAPs prepared by the present method are the alkalimetal acrylate-type SAPs obtained, for example, by copolymerizing 100parts of a monomer mixture comprising about 1 to about 50 mole percentacrylic acid, about 50 to about 99 mole percent of an alkali metalacrylate, and about 0.1 to about 5 percent by weight of an internalcrosslinking agent, in an aqueous solution containing at least about 15%and up to about 60%, and preferably about 20% to about 50%, by weight,of monomers. This is a preneutralized monomer mixture. In anotherpreferred embodiment, the alkali metal acrylate-type SAPs are obtainedby first solution polymerizing acrylic acid, then neutralizing the SAPhydrogel with an alkali metal base, i.e., a postneutralizationpolymerization.

As set forth above, polymerization of acidic or basic monomers, andoptional copolymerizable monomers, most commonly is performed by freeradical processes in the presence of a polyfunctional crosslinkingagent. The acidic and basic SAPs are cross-linked to a sufficient extentsuch that the SAP is water insoluble. Crosslinking renders the SAPssubstantially water insoluble, and, in part, serves to determine theabsorption capacity of the SAPs. For use in absorption applications, anacidic or basic SAP is lightly crosslinked, i.e., has a crosslinkingdensity of less than about 20%, preferably less than about 10%, and mostpreferably about 0.01% to about 7%.

A crosslinking agent most preferably is used in an amount of less thanabout 7 wt %, and typically about 0.1 wt % to about 5 wt %, based on thetotal weight of monomers. Examples of crosslinking polyvinyl monomersinclude, but are not limited to, polyacrylic (or polymethacrylic) acidesters, represented by the following formula (I), and bisacrylamides,represented by the following formula (II).

wherein X is ethylene, propylene, trimethylene, cyclohexylene,hexamethylene, 2-hydroxypropylene, —(CH₂CH₂O)_(p)CH₂CH₂—, or

p and r each are an integer 5 to 40, and k is 1 or 2;

wherein 1 is 2 or 3.

Specific crosslinking monomers include, but are not limited to,1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,3-butyleneglycol diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycoldiacrylate, diethylene glycol dimethacrylate, ethoxylated bisphenol Adiacrylate, ethoxylated bisphenol A dimethacrylate, ethylene glycoldimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanedioldimethacrylate, neopentyl glycol dimethacrylate, polyethylene glycoldiacrylate, polyethylene glycol dimethacrylate, triethylene glycoldiacrylate, triethylene glycol dimethacrylate, tripropylene glycoldiacrylate, tetraethylene glycol diacrylate, tetraethylene glycoldimethacrylate, dipentaerythritol pentaacrylate, pentaerythritoltetraacrylate, pentaerythritol triacrylate, trimethylolpropanetriacrylate, trimethylolpropane trimethacrylate,tris(2-hydroxyethyl)isocyanurate triacrylate,tris(2-hydroxyethyl)isocyanurate trimethacrylate, divinyl esters of apolycarboxylic acid, diallyl esters or a polycarboxylic acid, triallylterephthalate, diallyl maleate, diallyl fumarate,hexamethylenebismaleimide, trivinyl trimellitate, divinyl adipate,diallyl succinate, a divinyl ether of ethylene glycol, cyclopentadienediacrylate, tetraallyl ammonium halides, or mixtures thereof. Compoundssuch as divinylbenzene and divinyl ether also can be used ascrosslinking agents. Especially preferred crosslinking agents areN,N′-methylenebisacrylamide, N,N′-methylenebismethacrylamide, ethyleneglycol dimethacrylate, and trimethylolpropane triacrylate.

In the preparation of a color-stable SAP of the present invention, themonomers, for example, an α,β-unsaturated carboxylic acid, andespecially, acrylic acid, and crosslinking agent are subjected to apolymerization reaction in the presence of a polymerization initiator.One or more polymerization initiator is added to a mixture of themonomers and crosslinking agent to facilitate polymerization andformation of the SAP hydrogel.

Often the initiator comprises at least one thermal initiator and atleast one redox initiator. Any of the various polymerization initiatorsthat are known for use in preparing SAPs can be used in the presentinvention. However, in accordance with an important feature of thepresent invention, the polymerization initiator comprises a sulfinicacid derivative, such as 2-hydroxy-2-sulfinatoacetic acid, a saltthereof, or a mixture thereof, and is essentially free of acolor-forming oxidizing agent, like a persulfate, and a color-formingreducing agent, like ascorbic acid, isoascorbic acid, and sodiumerythrobate. As used herein, for color-forming reducing agents, the term“essentially free” is defined a total concentration of color-formingreducing agents of 0 ppm up to 10 ppm, by weight of the monomer mixture.For color-forming oxidizing agents, the term “essentially free” isdefined as less than 500 ppm, preferably less than 300 ppm, and, andmost preferably 0 ppm. In preferred embodiments, the polymerizationinhibitor is free of color-forming reducing agents and color-formingoxidizing agents, especially a persulfate.

Examples of useful polymerization initiators are redox initiatorscomprising (a) a reducing agent comprising a sulfinic acid derivative,such as 2-hydroxy-2-sulfinatoacetic acid, a salt thereof, or a mixturethereof, and optionally, 2-hydroxy-2-sulfonatoacetic acid, a sulfite orbisulfite of an alkali metal, ammonium sulfite, ammonium bisulfite,sodium metabisulfite, a sugar, an aldehyde, a primary or secondaryalcohol, and (b) an oxidizing agent, like hydrogen peroxide; an alkylhydroperoxide, like t-butyl hydroperoxide; t-butyl perbenzoate; t-butylperoxy isopropyl carbonate;1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane; benzoyl peroxide,dicumyl peroxide; caprylyl peroxide; sodium peracetate; or otheroxidizing agents known to persons skilled in the art. In preferredembodiments, the reducing agent is 2-hydroxy-2-sulfinatoacetic acid, andthe oxidizing agent is hydrogen peroxide.

Sulfinic acid derivatives useful as the reducing agent are disclosed inU.S. Pat. No. 6,211,400, incorporated herein by reference. The sulfinicacid derivatives have a general structural formula:

wherein M is hydrogen, an ammonium ion, or a monovalent or a divalentmetal ion of groups Ia, IIa, IIb, IVa, and VIIIb of the Periodic Tableof the Elements;

-   R¹ is OH or NR⁴R⁵, wherein R⁴ and R⁵, independently, are H or    C₁-C₆alkyl;-   R² is H or an alkyl, alkenyl, cycloalkyl, or aryl group, optionally    having 1-3 substituents independently selected from the group    consisting of C₁-C₆alkyl, OH, O—C₁-C₆alkyl, halogen, and CF₃; and-   R³ is COOM, SO₃M, COR⁴, CONR⁴R⁵, or COOR⁴, wherein M, R⁴, and R⁵ are    as defined above, or, if R² is unsubstituted or substituted aryl, R³    is H;    and the salts thereof.

In preferred embodiments, M is hydrogen, ammonium, an alkali metal ion,an alkaline earth metal ion, or zinc ion; R² is a hydrogen atom, or analkyl or aryl group, unsubstituted or substituted, independently, withone or two hydroxyl or alkoxy substituents; and R³ is COOM. Examples ofuseful sulfinic acid derivatives include, but are not limited to,

wherein M is H, Na, K, Mg, Ca, or Zn, and R⁴ is CH₃ or C₂H₅.

-   2-Hydroxy-2-sulfinatoacetic acid is a reducing agent having the    structure

and preferably is used as a salt, such as the disodium salt, e.g.,

2-Hydroxy-2-sulfinatoacetic acid is available commercially in pure formas BRUGGOLITE® FF7, and also as BRUGGOLITE® FF6, both from BrüggemannChemical, Heilbron, Germany. BRUGGOLITE® FF6 contains, by weight, 50-60%2-hydroxy-2-sulfinatoacetic acid disodium salt, 30-35% sodium sulfite(Na₂SO₃), and 10-15% 2-hydroxy-2-sulfonatoacetic acid disodium salt,i.e.,

2-Hydroxy-2-sulfonatoacetic acid also performs as a reducing agent, andcan be used in combination with 2-hydroxy-2-sulfinatoacetic acid.

The 2-hydroxy-2-sulfinatoacetic acid is free of formaldehyde and avoidsimparting a yellow color to an SAP resulting from the solutionpolymerization. In addition, 2-hydroxy-2-sulfinatoacetic acid and2-hydroxy-2-sulfonatoacetic acid decompose to the innocuous by-productssodium bisulfate, sodium formate, and carbon dioxide, thereby avoidingenvironmental and toxicologic problems.

A preferred redox initiator comprises 2-hydroxy-2-sulfinatoacetic aciddisodium salt, 2-hydroxy-2-sulfonatoacetic acid disodium salt, or amixture thereof, as the reducing agent, and hydrogen peroxide as theoxidizing agent, each used, for example, in an amount of about 2×10⁻⁵ toabout 2×10⁻² mole percent, based on moles of monomers (i.e., monomer andcrosslinking agent) present in the monomer mixture.

The redox initiators can be used singly or in suitable combination witha thermal initiator. Examples of suitable thermal initiators are the“azo” initiators, including, but not limited to, azobisisobutyronitrile;4-t-butylazo-4′-cyanovaleric acid; 4,4′-azobis(4-cyanovaleric acid);2,2′-azobis(2-amidinopropane) dihydrochloride;2,2′-azobis(2,4-dimethylvaleronitrile); dimethyl 2,2′-azobisisobutyrate;2,2′-azodimethyl bis(2,4-dimethylvaleronitrile);(1-phenylethyl)azodiphenylmethane; 2,2′-azobis(2-methylbutyronitrile);1,1′-azobis(1-cyclohexanecarbonitrile);2-(carbamoylazo)isobutyronitrile;2,2′-azobis(2,4,4-trimethylpenta-2-phenylazo-2,4-dimethyl-4-methoxyvaleronitrile;2,2′-azo-bis(2-methylpropane);2,2′-azobis(N,N′dimethyleneisobutyramidine) dihydrochloride;4,4′azobis(4-cyanopentanoic acid);2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide);2,2′-azobis(2-methyl-N-[1,1-bis(hydroxymethyl)ethyl]propionamide);2,2′-azobis[2-methyl-N(2-hydroxyethyl)propionamide];2,2′-azobis(isobutyramide) dihydrate; and other thermal initiators knownto persons skilled in the art.

Especially preferred polymerization initiators comprise (a) a redoxinitiator comprising (i) hydrogen peroxide as an oxidizing agent and(ii) 2-hydroxy-2-sulfinatoacetic acid, optionally2-hydroxy-2-sulfonatoacetic acid, salts thereof, or a mixture thereof asthe reducing agent, and (b) an azo initiator, such asazobisisobutyronitrile or 2,2′-azobis(2amidinopropane) dihydrochloride.A preferred thermal initiator for use in the present method is2,2′-azobis(2-amidinopropane) dihydrochloride, commercially availableunder the tradename V-50 from Wako Chemicals U.S.A., Inc., Richmond, Va.The initiator typically is used in an aqueous solution, but theinitiator can be diluted with another suitable solvent.

In addition to the polymerizable monomer, crosslinking agent, andpolymerization initiator, a monomer mixture used in the method of thepresent invention optionally contains a photoinitiator. Thephotoinitiator is present in the monomer mixture in a low amount, inparticular, in an amount of 0 to about 1000 ppm, by weight of monomersand cross-linking agent, and preferably about 10 to about 500 ppm. Toachieve the full advantage of the present invention, the photoinitiatoris present in the monomer mixture in an amount of about 15 to about 300ppm, by weight of monomers and crosslinking agent.

The photoinitiator is present to assist in reducing the residual acrylicacid monomer in the SAP. In prior methods of manufacturing an SAP, apersulfate was utilized as a component of the redox initiator, or as athermal initiator, to initiate the polymerization reaction and to reducethe amount of residual acrylic acid monomer in the SAP. However, apersulfate interacts with the MEHQ inhibitor present in acrylic acidmonomer and provides a slightly colored SAP. This color increases overtime, and especially in hot, humid conditions, to provide anunacceptable, honey-brown colored SAP.

In accordance with an important feature of the present invention, themonomer mixture is essentially free of a color-producing oxidizingagent, especially a persulfate. The photoinitiator, in combination witha low dose of UV radiation, then serves to reduce the amount of residualacrylic acid monomer in the SAP, i.e., to below 500, and preferablybelow 400 ppm by weight of the SAP, that previously was accomplishedusing a persulfate.

The identity of the photoinitiator is not limited, but is inert (i.e.,is not decomposed) under polymerization conditions. A preferredphotoinitiator has a structure

wherein R¹ and R², independently, are C₁₋₃ alkyl, or are taken togetherto form a C₄₋₈ carbocyclic ring, R³ is H, methyl, ethyl, or(OCH₂CH₂)_(n)OH, and n is 1-20.

Specific photoinitiators include, but are not limited to,

i.e., 1-(4-(2-hydroxyethyl)phenyl)-2-hydroxyl-2-methyl-1-propane-1-one,available as IRGACURE® 2959 from Ciba Specialty Chemicals, Hawthorne,N.Y.;

i.e., 1-phenyl-2-hydroxy-2-methyl-1-propane-1-one, also available fromCiba Specialty Chemicals as DAROCUR® 1173;hydroxycyclohexyl phenyl ketone, available from Ciba ChemicalSpecialties as IRGACURE® 184;

-   2,2-dimethoxy-1,2-diphenylethan-1-one, available from Ciba Specialty    Chemicals as IRGACURE® 651;    and mixtures thereof.

Additional useful photoinitiators include, but are not limited to,benzoin, benzoin ethers, benzyl ketals, acylphosphine oxides,camphorquinone, bisimidazole, a dialkylacetophenone, anα-aminoacetophenone, a chlorinated acetophenone, benzophenone, abenzophenone derivative (for example, p-benzoylbenzyl trimethyl ammoniumbromide), a thioxanthone derivative (for example,(3-(3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy)-2-hydroxypropyl)trimethyl ammonium chloride), and mixtures thereof.

In addition to the monomers, crosslinking agent, polymerizationinitiators, and photoinitiator, the monomer mixture contains water.Generally, the monomer mixture contains about 40 to about 80 percent,more preferably about 50 to about 70 percent, water, by weight of themonomer mixture.

The monomers present in monomer mixture are crosslinked concurrentlywith aqueous solution polymerization to a sufficient extent such thatthe resulting SAP is water insoluble, but has an ability to absorbseveral times its weight in water to form an SAP hydrogel. In manycases, after comminuting and drying of the SAP hydrogel, the resultingSAP is surface treated. Surface treatment results in surfacecrosslinking of the SAP particles. Surface treating an SAP enhances theability of the SAP to absorb and retain aqueous media under a load.

As understood in the art, a surface-treated SAP particle has a higherlevel of cross-linking in the vicinity of the particle surface than inthe particle interior. As used herein, “surface” describes theouter-facing boundaries of the SAP particle. For porous SAP particles,exposed internal surfaces also are included in the definition ofsurface.

In general, surface treating is achieved by contacting an SAP with asolution of a surface crosslinking agent to wet the outer surfaces ofthe SAP particles. Surface crosslinking and drying of the SAP particlesthen are performed, preferably by heating at least the wetted surfacesof the SAP particles. Surface treating also can be achieved by“annealing” (i.e., heating) the SAP particles at a sufficienttemperature for a sufficient time to provide surface crosslinks.

Typically, a solution of a surface crosslinking agent contains about0.01% to about 4%, and preferably about 0.4% to about 2%, by weight,surface crosslinking agent in a suitable solvent, for example, water oran alcohol. The solution can be applied as a fine spray onto the surfaceof freely tumbling SAP particles at a ratio of about 1:0.01 to about1:0.5 parts by weight SAP particles to solution of surface crosslinkingagent. The surface crosslinker is present in an amount of 0.001% toabout 5%, and preferably 0.005% to about 0.5%, by weight of the SAPparticles. To achieve the full advantage of the present invention, thesurface crosslinker is present in an amount of about 0.01% to about0.4%, by weight.

The crosslinking reaction and drying of the surface-treated SAPparticles are achieved by heating the surface-treated polymer at asuitable temperature, e.g., about 105° C. to about 200° C., andpreferably about 105° C. to about 180° C. However, any other method ofreacting the crosslinking agent to achieve surface crosslinking of theSAP particles, and any other method of drying the SAP particles, likemicrowave energy or the such, can be used.

Suitable surface crosslinking agents possess sufficient reactivity suchthat crosslinking occurs in a controlled fashion, preferably at atemperature of about 25° C. to about 180° C. Nonlimiting examples ofsuitable surface crosslinking agents include:

-   (a) polyhydroxy compounds, such as glycols and glycerol;-   (b) metal salts;-   (c) quaternary ammonium compounds;-   (d) a multifunctional epoxy compound, for example, ethylene glycol    diglycidyl ether, bisphenol A diglycidyl ether, and bisphenol F    diglycidyl ether;-   (e) an alkylene carbonate, such as ethylene carbonate or propylene    carbonate;-   (f) a polyaziridine, such as 2,2-bishydroxymethyl butanol    tris[3(1-aziridine propionate)];-   (g) a haloepoxy, such as epichlorohydrin;-   (h) a polyamine, such as ethylenediamine;-   (i) a polyisocyanate, such as toluene diisocyanate, isophorone    diisocyanate, methylene diisocyanate, xylene diisocyanate, and    hexamethylene diisocyanate;-   (j) dihalides and disulfonate esters, for example, compounds of the    formula Y—(CH₂)_(p)—Y, wherein p is a number from 2 to 12, and Y,    independently, is halo (preferably bromo), tosylate, mesylate, or    other alkyl or aryl sulfonate esters;-   (k) multifunctional aldehydes, for example, glutaraldehyde,    trioxane, paraformaldehyde, terephthaldehyde, malonaldehyde, and    glyoxal, and acetals and bisulfites thereof;-   (l) multifunctional carboxylic acids and esters, acid chlorides, and    anhydrides derived therefrom, for example, di- and polycarboxylic    acids containing 2 to 12 carbon atoms, and the methyl and ethyl    esters, acid chlorides, and anhydrides derived therefrom, such as    oxalic acid, adipic acid, succinic acid, dodecanoic acid, malonic    acid, and glutaric acid, and esters, anhydrides, and acid chlorides    derived therefrom;-   (m) organic titanates, such as TYZOR AA, available from E.I. DuPont    de Nemours, Wilmington, Del.;-   (n) melamine resins, such as the CYMEL resins available from Cytec    Industries, Wayne, N.J.;-   (o) hydroxymethyl ureas, such as    N,N′-dihydroxymethyl-4,5-dihydroxyethylene urea;-   (p) a hydroxyalkylamide (HAA), for example, as disclosed in U.S.    Pat. No. 6,239,230, incorporated herein by reference, but not    limited to, bis[N,N-di(β-hydroxyethyl)] adipamide,    bis[N,N-di(β-hydroxypropyl)] succinamide,    bis[N,N-di(β-hydroxyethyl)] azelamide, bis[N—N-di(β-hydroxypropyl)]    adipamide, and bis[N-methyl-N-(β-hydroxyethyl)] oxamide. A    commercially available β-HAA is PRIMID™ XL-552 from EMS-CHEMIE,    Domat, Switzerland. Another commercially available HAA is PRIMID™    QM-1260 from EMS-CHEMIE;-   (q) 2-oxazolidinone and derivatives thereof; and-   (r) other crosslinking agents for SAPs known to persons skilled in    the art.

A preferred surface crosslinking agent comprises an HAA, ethylene glycoldiglycidyl ether (EGDGE), propylene glycol, or mixtures thereof.

It is theorized, but not relied upon herein, that inhibitors, which areadded to the vinyl monomers to prevent premature polymerization duringtransport and storage, and which are present in the SAP polymer, areslowly oxidized by a persulfate causing the color of the SAP to degradefrom white to honey brown. This color change occurs at a faster rate atelevated temperatures and relative humidity.

For example, the monomethyl ether of hydroquinone (MEHQ) is theinhibitor typically used to prevent the premature polymerization ofacrylic monomers used in the manufacture of the SAPs, like acrylic acidand the crosslinking agents. Typically, the amount of inhibitor, likeMEHQ, added to the monomer is about 15 to about 200 ppm. The inhibitorsare not consumed during polymerization and are present in the SAPhydrogel after polymerization of the monomers. The color change of theSAP is theorized to result from oxidation of an inhibitor, like MEHQ, bya persulfate, or other color-forming reducing agent, to a quinone.

To prevent color degradation of an SAP to a consumer-unacceptable honeybrown color, a monomer mixture used to provide an SAP comprises asulfinic acid derivative, preferably 2-hydroxy-2-sulfinatoacetic acid,optionally 2-hydroxy-2-sulfonatoacetic acid, salts thereof, or amixture, thereof, as a reducing agent of the redox initiator, isessentially free of a persulfate, and optionally includes about 10 toabout 1000 ppm of a photoinitiator. By using a monomer mixtureessentially free of a persulfate, the resulting SAP has a crisp, whitecolor, and the white color of the resulting SAP is stabilized andpreserved. The optional photoinitiator is present in the monomer mixtureto assist in reducing the amount of free monomer in the SAP.

Therefore, the present invention is directed to a method ofmanufacturing an SAP that improves the color, and avoids discoloration,of SAPs. The interaction between sodium persulfate and the MEHQinhibitor results in a low initial SAP color and additional SAPdiscoloration during storage, especially under hot and humid conditions.To date, sodium persulfate has been an important, or a necessary,ingredient in the monomer mixture to reduce the residual acrylic acidcontent in the SAP to a consumer-acceptable level.

It now has been found that the monomer mixture can be essentially freeof a persulfate, and a redox initiator of the present invention, incombination with the optional thermal initiator, optionalphotoinitiator, and optional UV radiation dose, can be used to reduceresidual acrylic acid in a color-stable SAP. In one particularembodiment of the present invention, a low dose of UV radiation can beadministered at the end of the polymerization, i.e., after a majority ofthe acrylic acid monomer has been converted to an SAP hydrogel and thewater content of the hydrogel preferably has been reduced to about 25 wt% or less. In accordance with the present invention, unexpectedly lowresidual acrylic acid levels are achieved and the initial color andlong-term color of the SAP are improved considerably.

In particular, a monomer mixture is formed by admixing the monomers,crosslinking agent, polymerization initiators, optional photoinitiator,and water. Although the order of admixing these materials is notparticularly important, it is preferred to add the initiators last forsafety reasons. The amounts of the individual components of the monomermixture are set forth above.

The monomer mixture then is subjected to conditions under which themonomers and crosslinking agents polymerize to form an SAP hydrogel. Theconditions can be static or continuous, for example, by applying themonomer mixture to a moving conveyor that passes through a heating zonethat initiates the polymerization reaction. After polymerization on theconveyor, the resulting SAP hydrogel optionally advances to a UV zone,wherein an optional low dose of UV radiation is applied to the SAPhydrogel. The SAP hydrogel also is subjected to a mechanicalcomminution, i.e., reduction of the SAP hydrogel to SAP hydrogelparticles, for example, by chopping, prior to or after optional UVtreatment. Preferably, the SAP hydrogel is comminuted after the optionalUV treatment.

The SAP hydrogel, if not previously neutralized, can be neutralized witha base, for example, with sodium carbonate, to provide an SAP hydrogelhaving a degree of neutralization (DN) of about 50 to about 100,preferably about 65 to about 85, more preferably about 75 to about 80.This postneutralization step, if necessary, can be performed before orafter the SAP hydrogel optionally is irradiated. Preferably, themonomers are neutralized prior to polymerization.

Drying of the SAP hydrogel provides a color-stable SAP of the presentinvention. The dehydration step can be performed by heating the SAPhydrogel at a temperature of about 120° C. for about 1 to about 2 hoursin a forced-air oven or by heating the SAP hydrogel overnight at atemperature of about 60° C. The dried SAP optionally then can be surfacecrosslinked with a surface crosslinker, like ethylene glycol diglycidylether or an HAA, for example.

The SAP hydrogel optionally is subjected to a low dose of UV radiationintensity, for example, 2000 milliwatt/cm² or less, preferably 500milliwatt/cm² or less, and to achieve the full advantage of the presentinvention, about 5 to about 200 milliwatt/cm².

The UV radiation dose generally is administered using a UV lamp with anintensity of about 100 to about 700 watts per inch (W/in), preferablyabout 400 to about 600 W/in, for 0.1 seconds to 10 minutes, with thedistance between the UV lamp and the SAP hydrogel being 2 to 30centimeters. UV radiation can be conducted under vacuum, in the presenceof an inorganic gas, such as nitrogen, argon, helium, and the like, orin air. Suitable UV sources include a UV flood system from Starna or aSolartell Solarscope, Model 1, with a multidirectional probe.

Particles of a color-stable SAP of the present invention can be in anyform, either regular or irregular, such as granules, fibers, beads,powders, flakes, or foams, or any other desired shape, such as a sheet.In embodiments wherein the color-stable SAP is prepared using anextrusion step, the shape of the SAP is determined by the shape of theextrusion die. The shape of the color-stable SAP particles also can bedetermined by other physical operations, such as milling.

In one embodiment, the particles of the color-stable SAP are in the formof a granule or a bead, having a particle size of about 10 to about10,000 microns (μm), and preferably about 100 to about 1,000 μm. Toachieve the full advantage of the present invention, the particles ofthe color-stable SAP have a particle size of about 150 to about 800 μm.

In another embodiment, the particles of the color-stable SAP are in theshape of a fiber, i.e., an elongated, acicular particle. The fiber canbe in the shape of a cylinder, for example, having a minor dimension(i.e., diameter) and a major dimension (i.e., length). The fiber alsocan be in the form of a long filament that can be woven. Suchfilament-like fibers have a weight of below about 80 decitex, andpreferably below about 70 decitex, per filament, for example, about 2 toabout 60 decitex per filament. Tex is the weight in grams per onekilometer of fiber. One tex equals 10 decitex. Polyacrylic acid is about4 decitex.

Cylindrical fibers of a color-stable SAP have a minor dimension (i.e.,diameter of the fiber) less than about 1 mm, usually less than about 500μm, and preferably less than 250 μm, down to about 50 μm. Thecylindrical fibers can have a relatively short major dimension, forexample, about 1 mm, e.g., in a fibrid, lamella, or flake-shapedarticle, but generally the fiber has a length of about 3 to about 100mm. The filament-like fibers have a ratio of major dimension to minordimension of at least 500 to 1, and preferably at least 1000 to 1, forexample, up to and greater than 10,000 to 1.

The method of the present invention also can be used in the preparationof a multi-component SAP, as disclosed in U.S. Pat. Nos. 6,072,101;6,159,591; 6,222,091; and 6,329,062, each incorporated herein byreference.

A color-stable SAP of the present invention has an outstandingwater-absorbing ability, and is useful for use in sanitary goods, paperdiapers, disposable diapers and similar hygienic goods, agricultural orhorticultural water-retaining agents, industrial dehydrating agents,sludge coagulants, thickening agents, condensation preventing agents forbuilding materials, release control agents for chemicals and variousother applications. Furthermore, a present color-stable SAP retains itswhite color over extended storage periods at elevated temperature andrelative humidity. The present color-stable SAP particles, therefore,are useful in articles, like diapers, having improved consumer appeal.

After storage at 60° C. and 90% relative humidity for 30 days, acolor-stable SAP of the present invention has an HC60 color value of atleast 60 and a maximum b-value of 10. To achieve the full advantage ofthe present invention, the color-stable SAP exhibits an HC60 color valueof at least 63 and a maximum b-value of 8 after storage at 60° C. and90% relative humidity for 30 days.

EXAMPLES

The following illustrate nonlimiting examples of the present invention,and are not intended to limit the scope thereof.

Test Methods

Centrifuge Retention Capacity (CRC)

The CRC (centrifuge retention capacity) test is designed to measure theamount of 0.9% saline solution retained inside an SAP under a specificcentrifuge force. Measurement of CRC is disclosed in U.S. Pat. No.6,187,828 and U.S. Pat. No. 5,633,316, each incorporated herein byreference.

Absorbency Under Load (AUL)

The AUL (absorbency under load) test is designed to measure the abilityof an SAP to absorb a fluid under load. Measurement of AUL is disclosedin U.S. Pat. No. 6,187,828 and U.S. Pat. No. 5,633,316, eachincorporated herein by reference.

Extractables

The extractable content of an SAP is determined by mixing 1.0 g of theSAP with 200 mL of a 0.9% saline solution in a 250 mL beaker containinga magnetic stirrer bar, then stirring the solution at 500 rpm for 16hours. The entire solution then is filtered. Fifty mL of the filtratethen is withdrawn, and the free acid groups of the polymer materialdissolved in the filtrate are titrated to pH 10 using a 0.1 mol/L NaOHsolution. The resulting solution then is titrated to pH 2.7 using a 0.1mol/L HCl solution to determine the concentration of neutralizedacrylates. The titration data are used to calculate the total amount ofextractable polymer present in the SAP, using a 0.9% saline solution asblank:

-   V_(a)=ml of base required to titrate the sample aliquot to pH 10.0-   V_(ab)=ml of base required to titrate the blank to pH 10.0-   V_(b)=ml of acid required to titrate the sample aliquot to pH 2.7-   V_(bb)=ml of acid required to titrate the blank to pH 2.7-   C_(a)=molar concentration of base (0.1 mol/L NaOH)-   C_(b)=molar concentration of acid (0.1 mol/L HCl)-   M_(a)=relative molar mass of acrylic acid, 72 g/mol-   M_(b)=relative molar mass of sodium acrylate, 94 g/mol-   D=dilution factor, 200/50=4-   m=mass of sample in g.

The amount of acrylic acid (e.g., polyacrylic acid) in the supernatantaliquot (N_(a)) is given by:N _(a)=(V _(a) −V _(ab))×(C _(a)/1000) (moles)

The total amount of acrylate in the supernatant aliquot (N₁) is givenby:N ₁=(V _(b) −V _(bb))×(C _(b)/1000) (moles)

The amount of neutralized acrylate in the supernatant aliqot (N_(b)) isgiven by:N _(b) =N ₁ −N _(a) (moles)

The relative amounts of acrylate acid groups (W_(a)) and sodium acrylategroups (W_(b)) are given by:W _(a) =N _(a) ×M _(a) ×D(g)W _(b) =N _(b) ×M _(b) ×D(g)

The extractable content (W) of the superabsorbent polymer is given by:W=((W _(a) +W _(b))/m)×100%(%).Residual Acrylic Acid

One gram (1 g) of SAP is weighed into a 250 mL beaker. Two hundred mL of0.9% saline solution are added, a stir bar is placed in the beaker, thebeaker is covered with parafilm, then the mixture is stirred at 500 rpmfor 1 hour. After 1 hour, the sample is allowed to settle for 5 minutesand the supernatant is filtered using a 3 cc sterile syringe and 0.45 μmfilter. The content of acrylic acid is measured by HPLC analysis using0.1 N sulfuric acid as mobile phase and UV detection (210 nm).

Hunter Color (HC60) and b-Value

This test procedure is a method of measuring the perceived color ofpolymer related to its spectral characteristics. Spectralcharacteristics are specified by reflectance (or transmittance) as afunction of wavelength. The measurement is performed on the polymerpowder using a MACBETH Color-Eye 2180 Spectrophotometer according to themanufacturer's instructions, using a reflection cuvette or 35×10 mmpetri dish with lid as a sample cell.

In this system,

“L” is a measure of the lightness of a sample, and ranges from 0 (black)to 100 (white), “b” is a measure of yellowness (positive b-values) orblueness (negative b-values), Hunter Color HC60 is defined as:HC60=L−3b.

Example 1

An aqueous monomer mixture containing 25.6 wt. % sodium acrylate, 7.4wt. % acrylic acid, 0.4 wt. % ethoxylated trimethylol propanetriacrylate based on monomer (i.e., sodium acrylate plus acrylic acid),0.025 wt. % 2-hydroxy-2-methyl-1-phenyl-propan-1-one (DAROCUR® 1173)based on monomer, 0.008 wt. % 2,2-dimethoxy-1,2-diphenylethan-1-one(IRGACURE® 651) based on monomer, and 0.02 wt. % a mixture containing50-60% 2-hydroxy-2-sulfinatoacetic acid, disodium salt, 30-35% sodiumsulfite, and 10-15% 2-hydroxy-2-sulfonatoacetic acid, disodium salt(BRUGGOLITE® FF6) based on monomer was prepared. The mixture was cooledto 15° C. and deoxygenated by bubbling a nitrogen stream through themixture. Polymerization of the resulting monomer mixture then wasinitiated by addition of 0.04 wt. % hydrogen peroxide based on monomerand, after 5 minutes, the mixture was placed under a UV light (UVintensity=20 mW/cm²) for 15 minutes. The resulting gel was extrudedthree times through a KitchenAid Model K5SS mixer fitted with a meatgrinder attachment. The gel was dried at 150° C. for one hour, thenmilled and sized to 150-850 μm. The dry powder then was surfacecrosslinked by spraying a solution, containing 0.1 wt. % ethylene glycoldiglycidyl ether based on powder, 3.35 wt. % water based on powder and1.65 wt. % propylene glycol based on powder onto the powder particles,followed by curing at 150° C. for one hour. The properties of theresulting polymer were as follows:

CRC = 33.2 g/g AUL 0.7 psi = 24.9 g/g Extractables =  9.8 wt. % Residualacrylic acid =  360 ppm Hunter Color, HC60, initial =   85 Hunter Color,HC60, after 30 days   75 @60° C./90% relative humidity = Hunter Color,b-value, initial =  2.6 Hunter Color, b-value, after 30 days  3.8. @60°C./90% relative humidity =

Comparative Example 1

An aqueous monomer mixture containing 25.6 wt. % sodium acrylate, 7.4wt. % acrylic acid, 0.4 wt. % ethoxylated trimethylolpropane triacrylatebased on monomer, 0.025 wt. % 2-hydroxy-2-methyl-1-phenyl-propan-1-one(DAROCUR® 1173) based on monomer, 0.008 wt. %2,2-dimethoxy-1,2-diphenylethan-1-one (IRGACURE® 651) based on monomer,and 0.02 wt. % ascorbic acid based on monomer was prepared. The mixturewas cooled to 15° C. and deoxygenated by bubbling a nitrogen streamthrough the mixture. Polymerization of the resulting monomer mixturethen was initiated by addition of 0.04 wt. % hydrogen peroxide based onmonomer and, after 5 minutes, the mixture was placed under a UV light(UV intensity=20 mW/cm²) for 15 minutes. The resulting gel was extrudedthree times through a KitchenAid Model K5SS mixer fitted with a meatgrinder attachment. The gel then was dried at 150° C. for one hour, thenmilled and sized to 150-850 μm. The dry powder then was surfacecrosslinked by spraying a solution containing 0.1 wt. % ethylene glycoldiglycidyl ether based on powder, 3.35 wt. % water based on powder, and1.65 wt. % propylene glycol based on powder onto the powder particles,followed by curing at 150° C. for one hour. The properties of theresulting polymer were as follows:

CRC = 33.4 g/g AUL 0.7 psi = 23.5 g/g Extractables = 11.5 wt. % Residualacrylic acid =  420 ppm Hunter Color, HC60, initial =   67 Hunter Color,HC60, after 30 days   51 @60° C./90% relative humidity = Hunter Color,b-value, initial =  6.4 Hunter Color, b-value, after 30 days 12.5. @60°C./90% relative humidity =

Comparative Example 2

An aqueous monomer mixture containing 25.6 wt. % sodium acrylate, 7.4wt. % acrylic acid, 0.4 wt. % ethoxylated trimethylolpropane triacrylatebased on monomer, 0.025 wt. % 2-hydroxy-2-methyl-1-phenyl-propan-1-one(DAROCUR® 1173) based on monomer, 0.008 wt. %2,2-dimethoxy-1,2-diphenylethan-1-one (IRGACURE® 651) based on monomer,and 0.25 wt. % sodium persulfate based on monomer was prepared. Themixture was cooled to 15° C. and deoxygenated by bubbling a nitrogenstream through the mixture. Polymerization of the resulting monomermixture then was initiated by placing it under a UV light (UVintensity=20 mW/cm²) for 20 minutes. The resulting gel was extrudedthree times through a KitchenAid Model K5SS mixer fitted with a meatgrinder attachment. The gel was dried at 150° C. for one hour, thenmilled and sized to 150-850 μm. The dry powder then was surfacecrosslinked by spraying a solution containing 0.1 wt. % ethylene glycoldiglycidyl ether based on powder, 3.35 wt. % water based on powder, and1.65 wt. % propylene glycol based on powder onto the powder particles,followed by curing at 150° C. for one hour. The properties of theresulting polymer were as follows:

CRC = 31.5 g/g AUL 0.7 psi = 23.2 g/g Extractables = 13.6 wt. % Residualacrylic acid =  280 ppm Hunter Color, HC60, initial =   55 Hunter Color,HC60, after 30 days   35 @60° C./90% relative humidity = Hunter Color,b-value, initial =  9.5 Hunter Color, b-value, after 30 days 14.8. @60°C./90% relative humidity =

Comparative Example 3

An aqueous monomer mixture containing 25.6 wt. % sodium acrylate, 7.4wt. % acrylic acid, 0.4 wt. % ethoxylated trimethylolpropane triacrylatebased on monomer, 0.025 wt. % 2-hydroxy-2-methyl-1-phenyl-propan-1-one(DAROCUR® 1173) based on monomer, 0.008 wt. %2,2-dimethoxy-1,2-diphenylethan-1-one (IRGACURE® 651) based on monomer,and 0.15 wt. % 2,2′-azobisamidinopropane dihydrochloride based onmonomer was prepared. The mixture was cooled to 15° C. and deoxygenatedby bubbling a nitrogen stream through the mixture. Polymerization of theresulting monomer mixture then was initiated by placing the mixtureunder a UV light (UV intensity=20 mW/cm²) for 20 minutes. The resultinggel was extruded three times through a KitchenAid Model K5SS mixerfitted with a meat grinder attachment. The gel was dried at 150° C. forone hour, then milled and sized to 150-850 μm. The dry powder then wassurface cross-linked by spraying a solution containing of 0.1 wt. %ethylene glycol diglycidyl ether based on powder, 3.35 wt. % water basedon powder and 1.65 wt. % propylene glycol based on powder onto thepowder particles, followed by curing at 150° C. for one hour. Theproperties of the resulting polymer were followed by:

CRC =  32.6 g/g AUL 0.7 psi =  22.8 g/g Extractables =  12.9 wt. %Residual acrylic acid =  3580 ppm Hunter Color, HC60, initial =   87Hunter Color, HC60, after 30 days   77 @60° C./90% relative humidity =Hunter Color, b-value, initial =  2.3 Hunter Color, b-value, after 30days  4.1. @60° C./90% relative humidity =

Example 1 and Comparative Examples 1-3 each are preneutralizationpolymerizations, and illustrate unexpected benefits provided by thepresent invention. Example 1, which utilized BRUGGOLIT® FF6 as areducing agent, provides an SAP having excellent properties, especiallywith respect to residual acrylic acid (RAA), extractables, and HC60 andb-values (both initial and after 30 days at 60° C. and 90% relativehumidity). Comparative Example 1, which utilized ascorbic acid as areducing agent, showed a substantial adverse effect on color (i.e., HC60value and b-value after 30 days at 60° C. and 90% relative humidity),and increased extractables and RAA compared to Example 1. ComparativeExample 2 utilized sodium persulfate, and demonstrated a very poorinitial color and substantial color degradation (i.e., HC60 value andb-values), and a substantial increase in RM compared to Example 1.Comparative Example 3 utilized only a thermal initiator, and althoughthe initial color and color stability was good, the RM was unacceptablefor commercial applications and the extractables increased substantiallycompared to Example 1.

Example 2

An aqueous monomer mixture containing 28 wt. % acrylic acid, 0.2 wt. %pentaerythritol triallyl ether based on acrylic acid, 0.025 wt. % sodiumpersulfate based on acrylic acid, 0.044 wt. %2-hydroxy-2-methyl-1-phenyl-propan-1-one (DAROCUR® 1173) based onacrylic acid, 0.022 wt. % 2,2-dimethoxy-1,2-diphenylethan-1-one(IRGACURE® 651), and 0.025 wt. % 2-hydroxy-2-sulfinatoacetic acid,disodium salt (BRUGGOLITE® FF7) was prepared. The mixture was cooled to15° C. and deoxygenated by bubbling a nitrogen stream through themixture. Polymerization of the resulting monomer mixture then wasinitiated by addition of 0.05 wt. % hydrogen peroxide based on monomerand, after 5 minutes, the mixture was placed under a UV light (UVintensity=20 mW/cm²) for 20 minutes. The resulting polyacrylic acid gelwas extruded through a KitchenAid Model K5SS mixer fitted with a meatgrinder attachment. Next, sodium carbonate was added to the hydrogel, toadjust the neutralization degree of the acrylic acid groups to 75 mol %followed by two additional extrusions. The gel was dried at 150° C. forone hour, then milled and sized to 150-850 μm. The dry powder then wassurface crosslinked by spraying a solution containing 0.15 wt. %ethylene glycol diglycidyl ether based on powder, 3.35 wt. % water basedon powder, and 8.0 wt. % methanol based on powder onto the powderparticles, followed by curing at 150° C. for one hour. The properties ofthe resulting polymer were as follows:

CRC = 30.4 g/g AUL 0.7 psi = 25.8 g/g Extractables =  8.2 wt. % Residualacrylic acid =  290 ppm Hunter Color, HC60, initial =   74 Hunter Color,HC60, after 30 days   66 @60° C./90% relative humidity = Hunter Color,b-value, initial =  3.9 Hunter Color, b-value, after 30 days  5.7. @60°C./90% relative humidity =

Comparative Example 4

An aqueous monomer mixture containing 28 wt. % acrylic acid, 0.2 wt. %pentaerythritol triallyl ether based on acrylic acid, 0.025 wt. % sodiumpersulfate based on acrylic acid, 0.044 wt. %2-hydroxy-2-methyl-1-phenyl-propan-1-one (DAROCUR® 1.173) based onacrylic acid, 0.022 wt. % 2,2-dimethoxy-1,2-diphenylethan-1-one(IRGACURE® 651) and 0.025 wt. % ascorbic acid was prepared. The mixturewas cooled to 15° C. and deoxygenated by bubbling a nitrogen streamthrough the mixture. Polymerization of the resulting monomer mixturethen was initiated by addition of 0.05 wt. % hydrogen peroxide based onmonomer and, after 5 minutes, the mixture was placed under a UV light(UV intensity=20 mW/cm²) for 20 minutes. The resulting polyacrylic acidgel was extruded through a KitchenAid Model K5SS mixer fitted with ameat grinder attachment. Next, sodium carbonate was added to thehydrogel to adjust the neutralization degree of the acrylic acid groupsto 75 mol %, followed by two additional extrusions. The gel was dried at150° C. for one hour, then milled and sized to 150-850 μm. The drypowder then was surface crosslinked by spraying a solution containing0.15 wt. % ethylene glycol diglycidyl ether based on powder, 3.35 wt. %water based on powder, 8.0 wt. % methanol based on powder onto thepowder particles, followed by curing at 150° C. for one hour. Theproperties of the resulting polymer were as follows:

CRC = 30.2 g/g AUL 0.7 psi = 25.4 g/g Extractables =  8.7 wt. % Residualacrylic acid =  315 ppm Hunter Color, HC60, initial   69 Hunter Color,HC60, after 30 days   48 @60° C./90% relative humidity = Hunter Color,b-value, initial =  4.3 Hunter Color, b-value, after 30 days 12.7. @60°C./90% relative humidity =

Comparative Example 5

An aqueous monomer mixture containing 28 wt. % acrylic acid, 0.2 wt. %pentaerythritol triallyl ether based on acrylic acid, 0.20 wt. % sodiumpersulfate based on acrylic acid, 0.044 wt. %2-hydroxy-2-methyl-1-phenyl-propan-1-one (DAROCUR® 1173) based onacrylic acid, and 0.022 wt. % 2,2-dimethoxy-1,2-diphenylethan-1-one(IRGACURE® 651) was prepared, then cooled to 15° C. and deoxygenated bybubbling a nitrogen stream through the mixture. Polymerization of theresulting monomer mixture then was initiated by placing the mixtureunder a UV light (UV intensity=20 mW/cm²) for 25 minutes. The resultingpolyacrylic acid gel was extruded through a KitchenAid Model K5SS mixerfitted with a meat grinder attachment. Next, sodium carbonate was addedto the hydrogel to adjust the neutralization degree of the acrylic acidgroups to 75 mol %, followed by two additional extrusions. The gel wasdried at 150° C. for one hour, then milled and sized to 150-850 μm. Thedry powder then was surface crosslinked by spraying a solutioncontaining 0.15 wt. % ethylene glycol diglycidyl ether based on powder,3.35 wt. % water based on powder, and 8.0 wt. % methanol based on powderonto the powder particles, followed by curing at 150° C. for one hour.The properties of the resulting polymer were as follows:

CRC = 29.7 g/g AUL 0.7 psi = 24.9 g/g Extractables = 10.0 wt. % Residualacrylic acid =  260 ppm Hunter Color, HC60, initial =   59 Hunter Color,HC60, after 30 days   38 @60° C./90% relative humidity = Hunter Color,b-value, initial =  9.8 Hunter Color, b-value, after 30 days 14.8. @60°C./90% relative humidity =

Comparative Example 6

An aqueous monomer mixture containing 28 wt. % acrylic acid, 0.2 wt. %pentaerythritol triallyl ether based on acrylic acid, 0.066 wt. %2-hydroxy-2-methyl-1-phenyl-propan-1-one (DAROCUR® 1173) based onacrylic acid, 0.033 wt. % 2,2-dimethoxy-1,2-diphenylethan-1-one(IRGACURE®651), and 0.10 wt. % 2,2′-azobisamidinopropane dihydrochloridewas prepared, then cooled to 15° C. and deoxygenated by bubbling anitrogen stream through the mixture. Polymerization of the resultingmonomer mixture then was initiated by placing the mixture under a UVlight (UV intensity=20 mW/cm²) for 25 minutes. The resulting polyacrylicacid gel was extruded through a KitchenAid Model K5SS mixer fitted witha meat grinder attachment. Next, sodium carbonate was added to thehydrogel to adjust the neutralization degree of the acrylic acid groupsto 75 mol %, followed by two additional extrusions. The gel was dried at150° C. for one hour and milled, then sized to 150-850 μm. The drypowder then was surface crosslinked by spraying a solution containing0.15 wt. % ethylene glycol diglycidyl ether based on powder, 3.35 wt. %water based on powder, and 8.0 wt. % methanol based on powder onto thepowder particles, followed by curing at 150° C. for one hour. Theproperties of the resulting polymer were as follows:

CRC =  30.4 g/g AUL 0.7 psi =  23.5 g/g Extractables =  12.4 wt. %Residual acrylic acid =  5870 ppm Hunter Color, HC60, initial =   76Hunter Color, HC60, after 30 days   68 @60° C./90% relative humidity =Hunter Color, b-value, initial =  3.5 Hunter Color, b-value, after 30days  5.3. @60° C./90% relative humidity =

Example 2 and Comparative Examples 4-6 each are postneutralizationpolymerizations, and, similar to Example 1 and Comparative Examples 1-3,illustrate unexpected benefits provided by the present invention.Example 2 and Comparative Examples 4-6 show the good initial color andcolor stability provided when a sulfinic acid derivative is used as thereducing agent of a redox initiator. In particular, color degradation inComparative Examples 4 and 5 was observed after storing the SAP at 60°C. and 90% relative humidity for 30 days. Comparative Example 5 alsoexhibits a relatively poor initial color. Comparative Example 6exhibited an unacceptable level of RAA for commercial applications and asubstantial increase in extractables compared to Example 2.

Example 3

A 10 L capacity polyethylene vessel, well insulated by foamed polymermaterial, was charged with 3400 g of demineralized water and 1400 g ofacrylic acid. N,N′-methylenbisacrylamide (2.8 g) then was added ascopolymerization crosslinker. At a temperature of 10° C., 2.2 g of2,2′-azobisamidinopropane dihydrochloride, dissolved in 25 g ofdemineralized water, and 0.28 g of hydrogen peroxide, dissolved in 150 gof demineralized water, were added in succession, and the mixture wasstirred. The resulting solution then was deoxygenated by bubbling anitrogen stream through the solution for 30 minutes, followed by theaddition of 0.2 g of a mixture containing 50-60%2-hydroxy-2-sulfinatoacetic acid, disodium salt, 30-35% of sodiumsulfite and 10-15% of 2-hydroxy-2-sulfonatoacetic acid, disodium salt(BRUGGOLITE® FF6), dissolved in 25 g of demineralized water. Thereaction solution then was allowed to stand without stirring, and thetemperature of the polymerization rose to about 96° C. A solid gel wasobtained, and the gel subsequently was comminuted mechanically. Next, a50% strength by weight sodium hydroxide solution was added to thehydrogel to adjust the neutralization degree of the acrylic acid groupsto 74 mol %, followed by two additional extrusions. The gel then wasdried, ground, and classified to a particle size distribution 106-850μm. One kilogram of the dried hydrogel then was sprayed in a plowsharemixer with a solution containing 25 g demineralized water, 25 gpropylene glycol, and 2.0 g N,N,N′,N′-tetrakis(2-hydroxyethyl)adipamide(PRIMID XL-522, commercially available from Ems-Chemie AG), then themixture was heated at 165° C. for 2 hours. The properties of theresulting polymer were as follows:

CRC = 28.4 g/g AUL 0.7 psi = 26.8 g/g Extractables =  4.3 wt. % Residualacrylic acid =  220 ppm Hunter Color, HC60, initial =   73 Hunter Color,HC60, after 30 days   65 @60° C./90% relative humidity = Hunter Color,b-value, initial =  6.2 Hunter Color, b-value, after 30 days  7.4. @60°C./90% relative humidity =

Comparative Example 7

A 10 L capacity polyethylene vessel, well insulated by foamed polymermaterial, was charged with 3400 g of demineralized water and 1400 g ofacrylic acid. N,N′-methylenbisacrylamide (2.8 g) added ascopolymerization crosslinker. At a temperature of 10° C., 2.2 g of2,2′-azobisamidinopropane dihydrochloride, dissolved in 25 g ofdemineralized water, and 3.5 g of sodium persulfate, dissolved in 150 gof demineralized water, were added in succession and the mixture wasstirred. The resulting solution then was deoxygenated by bubbling anitrogen stream through the solution for 30 minutes, followed by theaddition of 0.2 g of ascorbic acid, dissolved in 25 g of demineralizedwater. The reaction solution then was allowed to stand without stirring,and the temperature of the polymerization rose to about 94° C. A solidgel was obtained, and the gel subsequently was comminuted mechanically.Next, a 50% strength by weight sodium hydroxide solution was added tothe hydrogel to adjust the neutralization degree of the acrylic acidgroups to 74 mol %, followed by two additional extrusions. The gel thenwas dried, ground and classified to a particle size distribution 106-850μm. One kilogram of the dried hydrogel then was sprayed in a plowsharemixer with a solution containing 25 g demineralized water, 25 gpropylene glycol, and 2.0 g N,N,N′,N′-tetrakis(2-hydroxyethyl)adipamide(PRIMID XL-522), and the mixture was heated at 165° C. for 2 hours. Theproperties of the resulting polymer were as follows:

CRC = 28.2 g/g AUL 0.7 psi = 24.9 g/g Extractables =  6.5 wt. % Residualacrylic acid =  180 ppm Hunter Color, HC60, initial =   67 Hunter Color,HC60, after 30 days   34 @60 C/90% relative humidity = Hunter Color,b-value, initial =  6.4 Hunter Color, b-value, after 30 days 14.1. @60°C./90% relative humidity =

Example 3 and Comparative Example 7 are postneutralizationpolymerizations and demonstrate the improvement in HC60 and b-valueafter storage at 60° C. and 90% relative humidity for 30 days when asulfinic acid derivative is utilized as the reducing agent of a redoxinitiator system.

Example 4

Acrylic acid (92.4 g), 0.026 g trimethylolpropane triacrylate, and 87.2g demineralized water were admixed. Sodium carbonate (40.4 g) then wasadded, and the temperature of the monomer solution was maintained below30° C. during this neutralization reaction. Then, 0.081 g2,2′-azobisamidinopropane dihydrochloride and 0.054 g hydrogen peroxidewere admixed into the monomer mixture. The monomer mixture then washeated to 62° C. and poured into a pan. Next, a mixture (0.027 g)containing 50-60% of 2-hydroxy-2-sulfinatoacetic acid, disodium salt,30-35% of sodium sulfite and 10-15% of 2-hydroxy-2-sulfonatoacetic acid,disodium salt (BRUGGOLITE® FF6), dissolved in 5 g of demineralizedwater, was added to initiate the polymerization. Due to the heat ofpolymerization, the major part of the water evaporated during thereaction, and at the end of the polymerization, a polymer mass having aresidual moisture content of about 15 wt. % was obtained. The polymermass was dried in a drying oven at 120° C., milled, and classified to aparticle size distribution 106-850 μm. The dry powder then was surfacecrosslinked by spraying a solution containing 0.12 wt. % ethylene glycoldiglycidyl ether based on powder, 3.35 wt. % water based on powder, and1.65 wt. % isopropanol based on powder onto the powder particles,followed by curing at 150° C. for one hour. The properties of theresulting polymer were as follows:

CRC = 31.8 g/g AUL 0.7 psi = 22.6 g/g Residual acrylic acid = 415 ppmHunter Color, HC60, initial = 88 Hunter Color, HC60, after 30 days = 65@60° C./90% relative humidity Hunter Color, b-value, initial =  1.8Hunter Color, b-value, after 30 days =  4.8. @60° C./90% relativehumidity

Example 5

Acrylic acid (92.4 g), 0.026 g trimethylolpropane triacrylate, and 87.2g demineralized water were admixed. Sodium carbonate (40.4 g) was added,and the temperature of the monomer solution was maintained below 30° C.during this neutralization reaction. Then, 0.081 g2,2′-azobisamidinopropane dihydrochloride, 0.018 g DAROCUR® 1173, and0.054 g hydrogen peroxide were admixed into the monomer mixture. Themonomer mixture was heated to 62° C. and poured into a pan, then amixture (0.027 g) containing 50-60% 2-hydroxy-2-sulfinatoacetic acid,disodium salt, 30-35% sodium sulfite, and 10-15%2-hydroxy-2-sulfonatoacetic acid, disodium salt (BRUGGOLITE® FF6),dissolved in 5 g of demineralized water, was added to initiate thepolymerization. Due to the heat of polymerization, the major part of thewater evaporated during the reaction, and at the end of thepolymerization, a polymer mass having a residual moisture content ofabout 15 wt. % was obtained. The polymer mass was placed under UV light(UV intensity=20 mW/cm²) for 8 minutes, then dried in a drying oven at120° C., milled, and classified to a particle size distribution 106-850μm. The dry powder then was surface crosslinked by spraying a solutioncontaining 0.12 wt. % ethylene glycol diglycidyl ether based on powder,3.35 wt. % water based on powder, and 1.65 wt. % isopropanol based onpowder onto the powder particles, followed by curing at 150° C. for onehour. The properties of the resulting polymer were as follows:

CRC = 30.7 g/g AUL 0.7 psi = 22.9 g/g Residual acrylic acid = 65 ppmHunter Color, HC60, initial = 86 Hunter Color, HC60, after 30 days 64@60° C./90% relative humidity = Hunter Color, b-value, initial =  2.0Hunter Color, b-value, after 30 days  5.2. @60° C./90% relative humidity=

Comparative Example 8

Acrylic acid (92.4 g), trimethylolpropane triacrylate, and 87.2 g ofdemineralized water were mixed together. Sodium carbonate (40.4 g) wasadded, and the temperature of the monomer solution was maintained below30° C. during this neutralization reaction. Then, 0.081 g2,2′-azobisamidinopropane dihydrochloride and 0.15 g sodium persulfatewere admixed into the monomer mixture. The monomer mixture was pouredinto a pan and heated to 67° C. to initiate the polymerization. Due tothe heat of polymerization, the major part of the water evaporatedduring the reaction, and at the end of the polymerization, a polymermass having a residual moisture content of about 13 wt. % was obtained.The polymer mass was dried in a drying oven at 120° C., milled, andclassified to a particle size distribution 106-850 μm. The dry powderthen was surface crosslinked by spraying a solution, containing 0.12 wt.% ethylene glycol diglycidyl ether based on powder, 3.35 wt. % waterbased on powder, and 1.65 wt. % iso-propanol based on powder onto thepowder particles, followed by curing at 150° C. for one hour. Theproperties of the resulting polymer were as follows:

CRC = 32.3 g/g AUL 0.7 psi = 20.8 g/g Residual acrylic acid = 175 ppmHunter Color, HC60, initial = 65 Hunter Color, HC60, after 30 days 22@60° C./90% relative humidity = Hunter Color, b-value, initial =  7.9Hunter Color, b-value, after 30 days 14.5. @60° C./90% relative humidity=

Examples 4 and 5 and Comparative Example 8, all preneutralizationpolymerizations, show that the optional UV dose substantially reducesRAA without adversely affecting initial color or color stability(Examples 4 and 5). Comparative Example 8 shows a poor color stabilityand low initial b-value when a persulfate is used as a component of theinitiator system and a sulfinic acid derivative is omitted (ComparativeExample 8 compared to Examples 4 and 5).

Overall, it has been found that essentially eliminating a persulfate,and including 2-hydroxy-2-sulfinatoacetic acid, a sulfinic acidderivative, preferably 2-hydroxy-2-sulfonatoacetic acid, salts thereof,or a mixture thereof as the reducing agent of a polymerization initiatorin the monomer mixture, and optionally irradiating the resulting SAPhydrogel with a low dose of UV radiation provides an SAP manufacturingprocess for a color-stable SAP. The method applies to bothpreneutralization and postneutralization SAP manufacturing processes.The color-stable SAP maintains a crisp, white color over extendedstorage periods, including in high temperature and humidity storageconditions.

Many modifications and variations of the invention as hereinbefore setforth can be made without departing from the spirit and scope thereofand, therefore, only such limitations should be imposed as are indicatedby the appended claims.

1. A method of manufacturing color-stable superabsorbent polymerparticles comprising the steps of: (a) forming a monomer mixturecomprising: (i) at least one monomer capable of forming a superabsorbentpolymer, (ii) a crosslinking agent, (iii) an initiator system comprisinga sulfinic acid derivative having a structure

wherein M is hydrogen, an ammonium ion, or a monovalent or a divalentmetal ion of groups Ia, IIa, IIb, IVa, and VIIIb of the Periodic Tableof the Elements; R¹ is OH or NR⁴R⁵, wherein R⁴ and R⁵, independently,are H or C₁-C₆alkyl; R² is H or an alkyl, alkenyl, cycloalkyl, or arylgroup, optionally having 1-3 substituents independently selected fromthe group consisting of C₁-C₆alkyl, OH, O—C₁-C₆alkyl, halogen, and CF₃;and R³ is COOM, SO₃M, COR⁴, CONR⁴R⁵, or COOR⁴, wherein M, R⁴, and R⁵ areas defined above, or, if R² is unsubstituted or unsubstituted aryl, R³is H; and the salts thereof; or a mixture thereof; (iv) an optionalphotoinitiator, and (v) water; (b) polymerizing the monomer and thecrosslinking agent in the monomer mixture to form a superabsorbentpolymer hydrogel; (c) optionally subjecting the superabsorbent polymerhydrogel to a low dose of UV radiation; (d) comminuting thesuperabsorbent polymer hydrogel to provide superabsorbent hydrogelparticles; and (e) drying the superabsorbent polymer hydrogel particlesfor a sufficient time at a sufficient temperature to provide thecolor-stable superabsorbent polymer particles, wherein the color-stableSAP particles, after storage for 30 days at 60° C. and 90% relativehumidity, exhibit an HC60 color value of at least 60 and a maximumb-value of
 10. 2. The method of claim 1 wherein optional step (c) isperformed prior to step (d).
 3. The method of claim 1 wherein step (d)is performed prior to optional step (c).
 4. The method of claim 1wherein the initiator system further comprises a thermal initiator. 5.The method of claim 1 wherein the initiator system comprises hydrogenperoxide as an oxidizing agent.
 6. The method of claim 5 wherein thehydrogen peroxide is present in the monomer mixture in a sufficientamount to initiate polymerization and avoid a substantial decrease insuperabsorbent polymer absorption properties.
 7. The method of claim 1wherein the sulfinic acid derivative comprises2-hydroxy-2-sulfinatoacetic acid, a salt thereof, or a mixture thereof.8. The method of claim 7 wherein the initiator system further comprises2-hydroxy-2-sulfonatoacetic acid, a salt thereof, or a mixture thereof.9. The method of claim 1 wherein the initiator system further comprisessodium sulfite.
 10. The method of claim 1 wherein the initiator systemcontains 0 to 500 ppm of a persulfate.
 11. The method of claim 10wherein the initiator system is essentially free of a persulfate. 12.The method of claim 1 wherein the optional photoinitiator is present inthe monomer mixture in an amount of about 10 to about 1000 ppm, byweight, of the monomer mixture.
 13. The method of claim 1 wherein theoptional photoinitiator comprises a compound having a formula

wherein R¹ and R², independently, are C₁₋₃alkyl, or are taken togetherto form a C₄₋₈carbocyclic ring, R³ is H, methyl, ethyl, or(OCH₂CH₂)_(n)OH, and n is 1-20.
 14. The method of claim 1 wherein theoptional photoinitiator comprises hydroxycyclohexyl phenyl ketone,2,2-dimethoxy- 1,2-diphenylethan- 1-one,

benzoin, a benzoin ether, a benzyl ketal, an acylphosphine oxide,camphorquinone, bisimidazole, a dialkylacetophenone, anα-aminoacetophenone, a chlorinated acetophenone, benzophenone, abenzophenone derivative, p-benzoylbenzyl trimethyl ammonium bromide, athioxanthone derivative,(3-(3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy)-2-hydroxypropyl)-trimethylammonium chloride, and mixtures thereof.
 15. The method of claim 1wherein the color-stable superabsorbent polymer particles comprise apolymerized α,β-unsaturated carboxylic acid, or a salt or anhydridethereof.
 16. The method of claim 1 wherein the monomer capable offorming the superabsorbent polymer is selected from the group consistingof acrylic acid, methacrylic acid, ethacrylic acid, α-chloroacrylicacid, α-cyanoacrylic acid, β-methylacrylic acid, α-phenylacrylic acid,β-acryloxypropionic acid, sorbic acid, α-chlorosorbic acid, angelicacid, cinnamic acid, p-chlorocinnamic acid, β-stearylacrylic acid,itaconic acid, citraconic acid, mesaconic acid, glutaconic acid,aconitic acid, maleic acid, fumaric acid, tricarboxyethylene, maleicanhydride, vinylsulfonic acid, allylsulfonic acid, vinyl toluenesulfonicacid, styrenesulfonic acid, sulfoethyl aceylate, sulfoethylmethacrylate, sulfopropyl acrylate, sulfopropyl methacrylate,sulfopropyl acrylate, sulfopropyl methacrylate,2-hydroxy-3-methacryloxypropyl sulfonic acid,2-acrylamido-2-methylpropane sulfonic acid, methacryloxy ethylphosphate, and mixtures thereof.
 17. The method of claim 1 wherein thesuperabsorbent polymer is selected from the group consisting ofpolyacrylic acid, a hydrolyzed starch-acrylonitrile graft copolymer, astarch-acrylic acid graft copolymer, a hydrolyzed acrylonitrilecopolymer, a hydrolyzed acrylamide copolymer, an ethylene-maleicanhydride copolymer, an isobutylene-maleic anhydride copolymer, apoly(vinylsulfonic acid), a poly(vinylphosphonic acid), apoly(vinylphosphoric acid), a poly(vinylsulfuric acid), a sulfonatedpolystyrene, and salts and mixtures thereof.
 18. The method of claim 1wherein the superabsorbent polymer comprises polyacrylic acidneutralized about 15% to 100%.
 19. The method of claim 1 wherein thesuperabsorbent polymer is selected from the group consisting of apoly(vinylamine), a poly(dialkylaminoalkyl(meth)acrylamide), apolyethylenimine, a poly(allylamine), a poly(allylguanidine), apoly(dimethyldiallylammonium hydroxide), a quaternized polystyrenederivative, a guanidine-modified polystyrene, a quaternizedpoly(meth)acrylamide) or ester analog, a poly(vinylguanidine), and saltsand mixtures thereof.
 20. The method of claim 1 wherein thesuperabsorbent polymer comprises a multicomponent superabsorbentpolymer.
 21. The method of claim 1 wherein the HC60 color value is atleast
 63. 22. The method of claim 1 wherein the maximum b-value is 8.23. The method of claim 1 wherein the color-stable SAP particles have aresidual monomer content of 500 ppm or less.
 24. The method of claim 1wherein the monomer comprises acrylic acid, a salt thereof, or a mixturethereof; the initiator system consists essentially of hydrogen peroxideas an oxidizing agent, 2-hydroxy-2-sulfinatoacetic acid, salts thereof,or mixtures thereof as a reducing agent, and a thermal initiator; theoptional photoinitiator is present in an amount of 0 to about 1000 ppm,by weight, of the monomer mixture; and the SAP hydrogel is subjected toUV radiation.
 25. The method of claim 24 wherein the initiator systemfurther comprises 2-hydroxy-2-sulfonatoacetic acid, sodium sulfite, or amixture thereof.
 26. The method of claim 1 further comprising the stepof: (f) surface treating the color-stable superabsorbent polymerparticles.
 27. Color-stable superabsorbent particles prepared by themethod of claim
 1. 28. An absorbent article comprising the color-stablesuperabsorbent particles prepared by the method of claim
 1. 29. Thearticle of claim 28 wherein the article is a diaper or a catamenialdevice.
 30. A diaper having a core, said core comprising at least 10% byweight of the color-stable superabsorbent particles prepared by themethod of claim
 1. 31. A diaper having a core, said core comprising atleast 40% by weight of the color-stable superabsorbent particlesprepared by the method of claim 1.